1. Field of the Invention
The present invention relates to infrared gas analysers particularly though not solely, for use in in-line sensing of the humidity of a humidified gas flow. An application of such an analyser is the sensing of humidity in humidified respiratory gases provided to a patient in a hospital, for example.
2. Description of the Prior Art
Present humidity sensors tend to have rather slow reaction times and are usually not suitable for use in high relative humidities. In the biomedical industry it is often desirable to measure the absolute humidity of a moving airstream which is at a high (60-100%) relative humidity, and with a fast response time.
An example of this application is the measurement of humidity in the breathing circuit of a patient on a ventilator. In this case it is desirable to know the humidity on a breath-by-breath basis or integral over a period of time, in a fast moving air flow, and without being affected by water condensation. The air stream in this system is warmer than the surrounding tube walls so there may be water droplets on the walls of the tube and in the air. It is also desirable to measure the humidity in the breathing circuit without the use of a sampling device (e.g. a suction pump or bypass tube). A device of this type is referred to as "mainstream" and has the advantages of speed, simplicity and quiet operation. Also, the humidity sensor must not affect the humidity being measured to any large extent.
The existing commercial devices for measuring humidity operate on several well known principles, which all have advantages and disadvantages.
Psychrometers are devices which measure the difference in temperature between a wet bulb and a dry bulb thermometer. They axe normally used for meteorology or in rooms or chambers where the humid air is static or slow moving and there is little chance of contamination.
There are many devices, made from polymer films or porous ceramics, which allow the water from humid air to adsorb onto their surface. This results in a change in the device's physical characteristics (e.g. resistance or capacitance), which can be measured. These devices are inexpensive and are popular for HVAC applications and consumer goods. The disadvantages of these devices are that their accuracy is low, long term drift and contamination of the adsorptive surface can occur, and they tend to get "waterlogged" at high relative humidities. This limits their application in a medical breathing circuit, where the relative humidity is often high.
Devices which measure the dew-point of humid air using a chilled mirror are expensive, but are well suited to high relative humidities and are accurate. Disadvantages of the technology are that the surface of the mirror is prone to contamination, and the device works best in still air.
There are commercial devices which measure the heat conductivity of humid air using heated temperature sensors. They are sensitive to the absolute humidity of the air and cope well with high humidifies. However, the measurement can be: affected by the air flow and by differing gas constituents. In a breathing circuit there may be high levels of oxygen or anaesthetic gases which would affect the humidity measurement.
One method of humidity measurement which gives a fast response time and could be made insensitive to variations in air speed and gas composition is infrared humidity sensing.